Quantum Enabled Science and Technology

QuEST (Quantum Enabled Science & Technology) is a Multi-Institutional Networked Programmme of Department of Science and Technology (DST), Government of India (GOI). It has 51 research projects grouped into four themes. QuEST Theme-I has 24 projects based on Quantum Information Technologies with Photonic Devices running at 17 top research Institutes of India.

Quantum Information Technologies with Photonic Devices is currently a very active area of research. It focuses on performing quantum information and quantum communication tasks using photons. In addition, this theme studies the foundations of quantum mechanics in general and quantum entanglement in particular in order to explore novel quantum information applications.

The photon is the first quantum entity to be proposed, as the quantum of energy. It plays an important role in the theory of blackbody radiation and Einstein’s theory of the photoelectric effect. While the photon is a key quantum mechanical concept to be applied to quantum technologies such as lasers and spectroscopy, and the emission and absorption of energy, most of the quantum technologies that emerged in the twentieth century involved semiconductors and the manipulation of electrons and atoms.The realization of the potential of a photon as a quantum particle and the consequences of the creation and manipulation of quantum light and its applications had to wait almost until the end of the twentieth century. The invention of lasers revolutionized twentieth century technology and the introduction of fiber optics brought in a very advanced technology for secure information communication at high speeds. Since 1980s, it became possible to generate and exploit quantum states of light, and photons provided the first system using which nonlocality of quantum mechanics was demonstrated.

Currently, quantum information science and technology is a very active field of research and photons are playing a major role in the establishment and development of the area of quantum information and quantum communication. The very first experiments on the violation of Bell inequalities were done with photonic two-qubit states. Although the initial aim of the Bell-inequality violation experiments was to prove that the local hidden variable interpretation of quantum mechanics is not possible, these experiments later helped establish quantum correlations as a resource that could be utilized for various practical applications. And now the research focus in this area is towards practical applications by exploiting quantum correlations in general and quantum entanglement in particular. To this end, there have been several theoretical and experimental works demonstrating the possibility of utilizing quantum correlations of photons for quantum information and quantum communication applications, such as quantum cryptography, quantum teleportation, remote state preparation, quantum sensing and quantum metrology.

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The photon-based quantum information and communication applications are the frontrunners in the race of quantum science and technology. The potential advantages of quantum metrology where measurement sensitivity is far beyond what is possible with classical resources were first proposed and realized with photons. Photons are especially useful for long distance quantum communication since their coherence time is very large and the photons can travel great distances and deliver the secure information to desired locations. As a result, most of the experimental implementations of quantum cryptography and quantum teleportation schemes have so far been based on photons. Although, a number of architectures are available to construct a quantum computer based on superconducting qubits, trapped ions, quantum dots etc., it has been recognized that only photons based schemes can be used for efficiently realizing the secure quantum communication protocols. It has also been recognized that even for the realizations of quantum computers based on ion-traps, superconductors, etc., optical and photon-based methods are needed to be developed for the control and manipulation of qubits. At present, several research groups and government organizations all over the world are working on realizing photonic quantum information and communication devices. Novel ideas such as post-selection and cluster state creation are leading to interesting and innovative developments in these efforts. The realization of quantum states of photons with orbital angular momentum has led to the realization of high-dimensional Hilbert spaces, which have their own set of unique advantages for secure communication applications such as quantum key distribution.

Despite their success, photonic quantum information and communication research faces several challenges, including controlled generation of the higher number-states of photons, availability of a perfect single-photon source and detector, a photonic quantum memory, and an efficient photon-number resolving detector. These challenges need to be overcome since the developments in photonics research are central to the advancement in quantum information science and technology. India has a long history of contributions in photonics research. On the theoretical front, there have been several groundbreaking works reported in the area of quantum optics in the last few decades, and on the experimental side, researchers have made several unique contributions in the area of classical optics and laser physics. More recently, several labs have started working in the area of photonic quantum information and quantum communication, and the community is poised at a very interesting juncture of taking the field forward. The new-generation experimentalists are undertaking very exciting research in this field. Given the successful space program in India, it is possible to have the country’s own satellite based quantum communication channel which is capable of performing continuous variable and discrete variable quantum key distribution using various degrees of freedom. Given the expertise in solid-state systems and superconductivity, it is possible to develop indigenous single-photon detectors and develop high-temperature quantum memory. QuEST has involved theorists as well as experimentalists coming together on a single platform and we are expecting breakthrough outcomes.

Quantum Information and Communication with Photons

The current state of photon-based quantum information processing is very much guided by the landmark experiments that have been performed in the last two decades. Some of these landmark experiments and ideas are:

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The seminal experimental work by Anton Zeilinger’s group in Vienna demonstrated quantum teleportation using photons for the first time. In quantum teleportation an arbitrary quantum state of a photon is transferred to a distant location without actually transferring the photon itself. Figure 1.1 describes the scheme. Two parties, Alice and Bob, share a pair of photons entangled in the polarization basis. Alice has another photon with an unknown state. In a quantum teleportation protocol,which involves Bob to make certain measurements based on the classical communication received from Alice, the unknown state of Alice’s photon is transferred to Bob’s photon. In the experiment by Zeilinger’s group, polarization-entangled photons from parametric down-conversion were used and quantum teleportation was demonstrated over a few meters.However,the later experiments demonstrated it over 143 kilometers. The quantum teleportation has now become an essential part of several quantum information protocols including quantum global internet.

The experimental work by Jennewien et al. proved to be the landmark experiment on quantum cryptography with photons. Cryptography is used for secure communication. A key is shared between the sender and the receiver; the sender encodes the message with the key and the receiver decodes the message with the same key. In classical key distribution, a binary string, known in-principle to everybody, is used for encryption and is communicated over an unprotected public channel. Currently, the key distribution problem is addressed by the widely- used RSA public key-private key protocol. This and an equivalence class of such methods exploit mathematical complexity involved in retrieving the secret private key by a third party. For example, the RSA method exploits the mathematical hardness of finding prime factors of large numbers. Such methods are not provably secure as they are vulnerable to technological advancement. If for instance a universal quantum computer were available, the problem of obtaining the unknown private key can be achieved in polynomial time by running the Shor algorithm on it.

In quantum cryptography, key distribution is achieved through the use of quantum principles and is therefore secure and called the quantum key distribution. Quantum key distribution protocols broadly fall into two categories --- those that use single photons of randomly chosen basis states, such as the BB84 protocol, or alternately those that use entangled photons, such as the Ekert protocol. The BB84 and the Ekert protocols are explained in Figure 1.2. In the BB84 protocol, the sender (Alice) randomly chooses between the two mutually unbiased bases (here denoted as HV and DA) and sends a state prepared in that basis to Bob as a ‘0’ or a ‘1’. After receiving the photon, Bob randomly chooses one of the two mutually unbiased bases and makes the measurement. In the Ekert protocol, Alice and Bob each receives one particle of the entangled pair. Alice then chooses a basis randomly and measures the state of her particle. After Alice’s measurement, Bob also chooses a basis randomly and measures the state of his particle. The two protocols are related in the sense that Alice’s “measuring a state” in the Ekert protocol is analogous to her “preparing a state” in the BB84 protocol. At the end of one round, Alice announces the choice of basis made by her for each photon, but not the bit value, over a public channel. Bob retains only those bits where he has made an identical choice and discards the others. This way Alice and Bob in principle share a private key. Furthe, post-processing steps are however required to obtain the privacy amplified, sifted key.
The experiment by Jennewien et al. was a convincing demonstration of quantum key distribution over 360 meters. The follow up experiments have now established quantum key distribution over more than 100 kilometers. The security in classical key distribution depends on the security of the communication channel. However, quantum key distribution is intrinsically secure because of the laws of quantum mechanics in the sense that if quantum key distribution succeeds, it is guaranteed to be secure. More recently cryptography with higher-dimensional states is gaining increased attention and the cryptographic protocols for two-dimensional variables have now been extended to higher- dimensional variables. One of the main advantages of high-dimensional quantum cryptography is the increased value of the maximum allowed transmission error.
There are limitations to QKD which include the facts that QKD over fiber is limited to a few tens of kilometers due to photon losses, is point-to-point and does not support complex network topologies. Furthermore, the current QKD rates are orders of magnitude lower than the volume of audio and video data transmitted. A lot of current research is focused on overcoming these limitations. Data transmission over a conventional single mode optic fiber typically results in a loss of ≈ 0.2 dB per km of fiber. This limits the range and key generation rates in fiber-based QKD. To overcome this problem one could use trusted nodes as classical repeater stations or use quantum repeaters (which are not yet available) or use free-space communication by connecting two ground stations via a satellite which could play the role of a trusted node. The world-over, QKD has been extensively developed and even commercially sold. The QuEST initiative will kick-start this activity and will hopefully result in the development of both fiber-based networks and satellite QKD for securing data in the future.

One of the applications of using entangled quantum states is in metrology and the experimental work by A. M. Steinberg’s group gave a big push forward to the research in this direction. The idea of quantum lithography was first theoretically proposed by Boto et al., in which it was shown that if one has N entangled photons, one can achieve N -fold enhancement in the lithographic resolution compared to the lithographic resolution one obtains using classical lithography at the same wavelength. In the work by A. M. Steinberg’s group, an entangled state consisting of three photons was prepared and thereby a three-fold enhancement in the fringe resolution was demonstrated.

Entanglement not only helps in transmitting quantum information but also helps in enhancing capacity of classical communication. For example, if Alice and Bob share a maximally entangled state then by sending a single qubit (half of the entangled pair) Alice can send two classical bits to Bob. Without entanglement only one classical bit can be transmitted by sending a single qubit. This is known as the dense coding and has been implemented using photonic setup.

In order to get an unknown qubit teleported from Alice to Bob, one needs to consume one maximally entangled pair and send two classical bits. However, if the state is known to Alice, in particular, she chooses a qubit either from the polar or the equatorial great circle of the Bloch sphere, it is possible to prepare its state at a remote location by using one maximally entangled pair and sending one classical bit. This reduces the classical communication cost and has been implemented by various groups using photonic qubits.

International Status

There are several active experimental groups in the world who are leading the way in photonics based quantum information processing. The group of Anton Zeilinger at the University of Vienna in Austria has made seminal contributions in the area of quantum information and communication using photons. On one hand, the group has experimentally investigated some of the foundational aspects of quantum mechanics and quantum entanglement. On the other hand, it has performed some of the very first experiments in the area of quantum cryptography, quantum teleportation, etc., that have truly established photonic quantum information and communication as a viable technology. Zeilinger’s group has actively been working on satellite based quantum communication and quantum entanglement in high-dimensional systems. The groups of Jian-Wei Pan at the University of Science and Technology in China and Thomas Jennewein at the Institute of Quantum Computing in Canada, who once worked in Anton Zeilinger’s group, are currently doing cutting-edge research in the area of quantum information and communication.

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While the group of Thomas Jennewein has more recently demonstrated a genuine three-particle entangled state, the group of Jian-Wei Pan has been leading a major research and development effort in China for achieving photonic quantum key distribution. Other than China, Japan and Singapore in Asia, a number of countries in EU, Canada and USA are already having intra-city or inter-city quantum communication links through fiber and free space. Some of them like Japan and China have recently demonstrated satellite quantum communication covering free space of 500-1200 kilometers. The group of Nicolas Gisin at the University of Geneva in Switzerland has done some foundational work in the area of long- distance quantum communication and quantum non-locality and is also currently leading the research in this area. The group of Philippe Grangier at the Laboratoire Charles Fabry in France in the past has performed some of the foundational experiments on quantum nonlocality and is currently working on continuous variable quantum cryptography and non-classical state generation. The group of Gerd Leuchs at the Max Planck Institute for the Science of Light in Germany has made several important contributions and has been working on quantum information processing, including quantum key distribution, quantum hacking and quantum cloning. In addition there are several other well-established experimental research groups that are very actively working in this area. Some of these groups are Paul Kwiat’s group at the University of Illinois and Urbana Champaign in USA, Robert Boyd’s group at the University of Ottawa in Canada, Aephraim Steinberg’s group at the University of Toronto in Canada, Dirk Bouwmeester’s group at the University of California at Santa Barbara in USA, Andrew White’s group at the University of Queensland in Australia.

National Status

The country has a valuable amount of expertise and presence in the fields of optics and quantum optics in theory as well as experiments. There are several research groups in India working on optical quantum computation and communication. A number of experimental groups are working towards implementing quantum key distribution in the lab using orbital angular momentum and polarization degrees of freedom of single photons. Some groups are working on quantum imaging and quantum non-locality related problems using single photons or non-classical light, and some of the PIs in QuEST have been directly involved with QKD experiments. There are several other groups who have contributed immensely to the theoretical research in the broad area of quantum information and quantum communication and are currently very active. Some of these groups have also been working very closely with the experimentalists.